Acoustic logging systems are routinely used in the oil and gas industry to measure formation acoustic properties of earth formation penetrated by a well borehole. These properties include the compressional and shear velocities of the formation, which are subsequently used to determine a variety of formation parameters of interest such as porosity and pore pressure. Additionally, acoustic logging systems are used to produce acoustic images of the borehole from which well conditions and other geological features can be investigated. Other applications of acoustic logging measurements include seismic correlation and rock mechanic determination.
The downhole instrument or borehole “tool” of an acoustic logging system typically comprises one or more sources of acoustic energy or “transmitters”, and one or more acoustic receivers. The transmitters and receivers are typically spaced axially on the body of the tool. A portion of the energy emitted by the one or more transmitters propagates through formation material surrounding the borehole, and is subsequently detected by the one or more receivers. Receiver response is then used to determine properties and parameters of interest.
A plurality of receivers can be azimuthally disposed at a given axial spacing thereby forming an “array” of receivers at that axial spacing. Depending on the type of measurement, each array may contain one or more azimuthally spaced receivers. Monopole measurements can be made with an array comprising one receiver. Dipole measurements require an array comprising at least two receivers.
Acoustic tools are required to be centered or “centralized” within the borehole to minimize the effect of tool standoff from the borehole wall. If the tool is decentralized or “eccentered” within the borehole, the acoustic waves traveling along the path of the short borehole distance arrive at the receivers sooner than those traveling along the long borehole path. This creates waveform smearing and distortion resulting in loss of coherence and poor data quality. Poor data quality is propagated to poor measures of formation properties or other parameters of interest
In wireline tools, acoustic receivers are typically disposed in the center of the tool and the tool is centralized in the borehole using mechanical centralizers. This arrangement effectively centralizes the receivers within the borehole which, in turn, tends to minimize adverse standoff effect on acoustic measurements in any borehole size in which tool centralization can be maintained.
In logging-while-drilling (LWD) tools, acoustic receivers are typically disposed on or near the perimeter of the tool, and the tool is preferably centralized within the borehole using wear bands that are slightly larger than the tool diameter. These fixed-diameter wear bands can only centralize the tool in certain borehole sizes. In different borehole sizes, which is common in drilling operations, eccentricity effects on acoustic LWD tool response can be severe.
Other techniques are used to minimize effects of tool eccentricity, especially in boreholes of varying diameter or in boreholes with irregular cross sections. One technique uses two receiver arrays, with the two receivers being azimuthally spaced at 180 degrees. Responses of the two receivers are averaged, or alternately summed, thereby yielding a composite signal with reduced adverse tool eccentricity effects. Another technique utilizes four azimuthally spaced receivers in each array, wherein the responses of the four receivers are again combined forming a composite signal with further reduce tool eccentricity effects. Manufacturing costs and operational reliability of tools comprising multiple receiver arrays at each axial spacing are considerably greater than tools comprising single receiver “arrays”.
Another system embodied to reducing tool eccentricity effects comprises a plurality of single acoustic receiver arrays combined with an ultrasonic transducer or “pinger”. The pinger measures the standoff of the tool as it rotates within the borehole. If the tool is equipped with at least three ultrasonic pingers, measures of borehole diameter and tool standoff can be made. Given these two measurements, the acoustic measurement can be made when the pinger system senses that the tool is centered in the borehole. Ultrasonic pingers add to the cost of the tool and introduce additional operational and reliability issues. Furthermore, the ultrasonic measurement quality is a function of borehole conditions including type of fluid filling the borehole. Unexpected changes in these borehole conditions, which are operationally common, can introduce errors in the eccentricity correction of the sensor responses.